Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 170843

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
343 AM EST Wed Jan 17 2018

A cold front will move southeast of the area later this morning with
snow likely across the forecast area. In the wake of the front, high
pressure will overspread the region later this evening and tomorrow
and drift offshore by early Saturday. Another cold front is expected
to approach the Carolinas early next week.


As of 330 AM EST Wednesday: Still trying to hit the moving target
early this morning, as once again the new model guidance has
continued on the trend toward more precip. The models now deepen
the upper trof to the point where a low closes off this morning
and moves across the mtns this afternoon, bringing even more
forcing to bear across the Piedmont of the Carolinas in the form
of upper divergence and mid-level frontogenesis. In addition, low
level frontogenesis along the Blue Ridge Escarpment will also come
into play as the cold front hangs up for the next few hours. What
is most interesting is how the guidance shows a blossoming area
of precip developing over the foothills before daybreak and
then expanding eastward over the Piedmont through the morning
hours. Radar imagery suggests this scenario is already underway
as we have seen an increasing trend near the Blue Ridge Escarpment
since about 07Z. The latest WPC guidance and SREF raise the precip
totals even higher than before across parts of the wrn Piedmont
and ern Upstate/metro CLT, to the point where they are solidly
in Warning criteria. We are most confident of reaching warning
criteria east of I-77. Fortunately all this agrees fairly well
with our current warned area. The exception might be Alexander
County NC and the rest of the NC foothills, which we will monitor
for an upgrade depending on how the snow piles up in the next
few hours. Confidence is lagging over the rest of Upstate SC as,
once again, the colder/drier air is having a difficult time being
dislodged from the north, west, and southwest. Some of our precip
will likely be used up bringing the wet bulb temp down low enough
for precip to change to snow. At the same time, the GSP metro
area will probably remain warmer than expected through daybreak,
in the lower 30s, so road conditions might be less likely to
be adversely affected. The warning will NOT be expanded further
across the Upstate or northeast GA as a result. On the back end,
the closing off of the upper low will slow the system down a bit,
so precip chances are tapered off a few hours later into the
afternoon over the eastern zones. This suggests we are ending
the warning for metro CLT too early. It will be extended until
6 pm in part to agree with the other warnings from the RAH and
CAE offices. Temps will be well below normal today. Meanwhile,
over the NC mountains, the wind will pick up and cold advection
will take hold, dropping wind chill down to Advisory criteria
over the higher elevations. Will issue a Wind Chill Advisory that
begins with the expiration of the Winter Weather Advisory, and
that will remain in effect overnight and into Thursday morning. As
for the black ice issue, we will certainly need to address it for
tonight/early Thursday, but prefer to act on that after we get a
feel for how much snow actually falls and after we clear out some
of the Advisories and Warnings. The tonight period should be mostly
clear and cold with fresh snow across parts of the area.


As of 230 AM EST Wednesday: The short term looks very quiet, as dry
quasi-zonal flow sets up across the eastern states. Dry sfc high
pres will settle in across the Deep South Thursday, lingering thru
Friday. Temps will start out cold Thursday morning, rebounding into
the 30s to mid 40s in the aftn under full sunshine. Melting snow may
limit max temps a category or so, but still expect good melting.
Whatever roads are wet will refreeze Thursday night, with temps
dipping into the upper teens to mid 20s. A warming trend will ensue
for Friday, with temps warming to near normal, mainly upper 40s
mountains and lower to mid 50s piedmont under sunny skies. Lows
Friday night under clear skies and light winds will be in the 20s.


As of 305 AM EST Wednesday: the extended forecast begins at 12z on
Saturday with broad upper ridging in place over the eastern 2/3 of
the CONUS and a deep upper trof over the West Coast. Over the next
12 to 24 hrs, the long-range models try to develop a closed upper
low over the far southeast and move it offshore by late Sunday.
On Monday, the previously mentioned West Coast trof will approach
the fcst area and likely take on a more negative tilt as it moves
over the area on Tuesday. At the sfc, dry high pressure will be
centered just off the SE Coast with low-lvl flow out of the SW.
As we move into Sunday, the high will slide farther offshore while
a low pressure system deepens over the Great Plains. The low is
expected to track NE towards the Great Lakes and bring a strong
cold front to our doorstep on Monday. Most of the model guidance
has the front east of the CWFA by early Tues with another round
of dry high pressure in its wake. As for the sensible fcst, no
major changes were needed with PoPs ramping up late Sunday and
into Monday. By early Tues, most of the precip is expected to be
east of the CWFA, however I did keep a chance for some light NW
flow snow showers over the NC/Tenn border region.


At KCLT and elsewhere: Still VFR across the Piedmont at issuance
time, but lower clouds and light snow are spreading across the
mtns. Expect a few more hours of VFR, so the onset was pushed
back closer to 08Z, but expect the precip (snow) and low clouds to
blossom right over the top of us in the 09Z to 12Z time frame. The
precip might still begin as a brief period of -RASN before
transitioning to -SN. As this happens, the cold front will move
across the region bringing the wind direction around to NW. Most
likely, an MVFR flight category will prevail with light snow, but
expect periods of IFR as the main band develops overhead in the 09Z
to 15Z time frame. Any bursts of heavy snow may bring with it LIFR
to VLIFR, but these bursts should be short-lived. New guidance is
lingering -SN well into the afternoon, so the snow restrictions were
tapered off later in the period. Still expecting an improvement to
VFR everywhere by the end of the period with -SN moving out. Wind
will be gusty from the NW this afternoon, especially at KAVL.

Outlook: Dry/VFR condition are expected to continue through the
week and into the weekend.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       Med   79%     Med   75%     Med   70%     High 100%
KGSP       High  82%     High  83%     High 100%     High 100%
KAVL       Med   76%     High  83%     High 100%     High 100%
KHKY       Med   65%     High  80%     High  90%     High 100%
KGMU       Med   68%     High  83%     High 100%     High 100%
KAND       Med   69%     High  91%     High  93%     High 100%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:


GA...Winter Weather Advisory until noon EST today for GAZ018-026-
     Winter Weather Advisory until 10 AM EST this morning for
NC...Winter Weather Advisory until noon EST today for NCZ035-
     Winter Storm Warning until noon EST today for NCZ068.
     Winter Weather Advisory until 7 AM EST this morning for NCZ051-
     Wind Chill Advisory from 10 AM this morning to 7 AM EST
     Thursday for NCZ048-051>053-058-059-062>065.
     Winter Weather Advisory until 10 AM EST this morning for
     Wind Chill Advisory from 10 AM this morning to 10 AM EST
     Thursday for NCZ033-049-050.
     Winter Storm Warning until 6 PM EST this evening for NCZ036-
SC...Winter Weather Advisory until noon EST today for SCZ001-002-
     Winter Storm Warning until noon EST today for SCZ003-006>008-
     Winter Storm Warning until 6 PM EST this evening for SCZ009-


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